The invention relates to a picture display device having a vacuum envelope which is provided with a transparent face plate with a luminescent screen for displaying pictures composed of pixels and with a rear wall, which display device comprises a plurality of sources for emitting electrons, electron transport ducts cooperating with the sources and having walls of a dielectric material for transporting, through vacuum, emitted electrons towards positions at a short distance from the luminescent screen, and means for accelerating the electrons towards the luminescent screen.
The display device described above is of the flat-panel type, as disclosed in EP-A-436997. Display devices of the flat-panel type are devices having a transparent face plate and, arranged at a small distance therefrom, a rear plate, with the inner surface of the face plate being provided with a pattern of phosphor dots or stripes. Electrons impinging upon the luminescent screen may be controlled to form a visual image which is visible via the front side of the face plate. The face plate may be flat or, if desired, curved (for example, spherical or cylindrical).
The display device described in EP-A-436997 comprises a plurality of juxtaposed sources for emitting electrons, local electron-propagation means cooperating with the sources and each constituting electron transport ducts, each having walls of a dielectric material having a secondary emission coefficient suitable for propagating emitted electrons, and an addressing system with electrodes (selection electrodes) which can be driven row by row for withdrawing electrons from the propagation means at predetermined extraction locations facing the luminescent screen, further means being provided for transporting extracted electrons towards the luminescent screen for producing an image composed of pixels.
The operation of the picture display device disclosed in EP-A-436997 is based on the recognition that electron propagation is possible when electrons impinge on a wall of a high-ohmic, substantially electrically insulating material (for example, glass or synthetic material) if an electric field of sufficient power is generated over a given length of the wall (by, for example, applying a potential difference across the ends of the wall). The impinging electrons then generate secondary electrons by wall interaction, which electrons are propagated towards a further wall section and in their turn generate secondary electrons again by wall interaction, and so forth.
Starting from the above-mentioned principle, a flat-panel picture display device can be realised by providing each one of a plurality of juxtaposed "compartments", which constitute propagation ducts, with a column of extraction apertures at a side which is to face a display screen. It will then be practical to arrange the extraction apertures along "horizontal" lines extending transversely to the ducts. By adding selection electrodes arranged in rows near to the arrangement of apertures, an addressing means is provided with which electrons can be selectively withdrawn from the "compartments" and directed towards the screen for producing a picture composed of pixels by activating respective areas of the luminescent screen.
The addressing system may be of the single-stage or of the multi-stage type.
EP-A-464937 particularly describes a multi-stage addressing system. A multi-stage addressing system using a number of preselection extraction locations, which number is a fraction of the number of pixels, and associated therewith a number of (fine-)selection apertures which corresponds to the number of pixels provides advantages with respect to the extraction efficiency and/or with respect to the complexity of the connections/driving circuitry. For controlling the preselection locations, a pattern of apertured preselection electrodes is used, and for controlling the fine-selection apertures a pattern of apertured fine-selection electrodes is used.
By withdrawing electrons at desired locations from the electron ducts and directing them towards the luminescent screen, a picture can be formed on the luminescent screen. In this case it is important that the electrons in the ducts do not have excessive velocities. If electrons having too high velocities during transport through the electron ducts would enter unaddressed selection apertures and reach the screen this could lead to loss of contrast of the picture on the screen. On the other hand, in the case of too high velocities, they might not be enter (miss or bypass) an addressed selection aperture and get lost so that a selected pixel on the luminescent screen would not be excited. Too high velocities may occur due to elastic collisions with the walls (back-scattering) or because electrons starting at a low velocity do not come into contact with the walls at all or do not come into contact with these walls until after they have covered a substantial distance (more than several millimeters) and gain more and more energy on their way. To prevent this, an "oblique" transport field may be applied having not only a longitudinal electric field component (E.sub.y) but also a transverse electric field component (E.sub.x), the latter pushing the electrons towards the non-apertured walls of the ducts. In a preferred embodiment, the electrons are pushed toward a rear wall of each duct which is opposite a front wall having the extraction apertures. It is thereby achieved that the electron current is confined to a longitudinal area proximate to the rear wall in particular. As it were, the electrons "hop" across the wall during transport, which has the envisaged effect.
A selection means is provided by providing the selection apertures with electrodes which can be energized by means of a first electric voltage so as to withdraw electron currents from the ducts via the apertures of a row, or they can be energized by means of a second "lower" electric voltage if no electrons should be locally withdrawn from the ducts. The electrons withdrawn from the ducts by this selection means can be transported towards the screen by applying an acceleration voltage. By providing a gradually, e.g. linearly, increasing potential across each rear duct wall and a similarly increasing, but lower potential across each duct wall having the extraction apertures, the field components E.sub.y and E.sub.x may be created. The rear wall potential may be defined by means of a high-ohmic resistance layer provided on the rear wall. For adjusting the rear wall potential the resistance layer is provided with electric contacts at longitudinally-spaced-apart portions of the transport duct. The potential at one contact, e.g. the contact closest to an electron-input end of the duct, should be adjusted carefully so as to obtain a uniform picture. The front wall potential can be adjusted, for example, by providing a plurality of parallel, strip-shaped electrodes on the screen side of the electron ducts, which electrodes can be given an approximately linearly increasing potential during operation. These electrodes may also be used to advantage for selecting an image line by providing apertures in these electrodes and connecting them to a circuit for providing a selection voltage.
In the display described above, suitable potentials force the electrons to "hop" along a wall. This means that each injected electron is incident on a wall and releases one secondary electron on average, which secondary electron is accelerated by the transport field, impinges upon the rear wall (or upon a side wall), and in its turn generates another secondary electron and so forth. When driven in such a mode, the number of electrons which can reach too high/excessive velocities is limited to a considerable extent.
Electrons which are withdrawn from the electron ducts can be transported towards (localized areas of) the luminescent screen by applying a sufficiently large voltage difference between the electron ducts and the screen, for example a difference of 3 kV. One image line at a time can thus be written. Video information (grey scales) can be presented, for example in the form of pulse width modulation. The distance between the extraction apertures and the screen may be very small so that the spots remain small. Electrons extracted from an individual aperture and accelerated towards the screen can be localized by providing an electron localization structure in the form of, for example, a structure of horizontal and/or vertical walls, or in the form of an apertured plate, between the extraction apertures and the luminescent screen.
In the above-described type of display, the transport mechanism appears to adjust automatically in normal operation, i.e. the wall charge on the insulating walls adapts itself. However, electron transport in a duct is sometimes unexpectedly impeded, or appears to start with difficulty, when the display is switched on, or after periods in which there has been (little or) no electron transport along a given duct location for a substantial period of time. This can adversely affect the presentation of an image on the screen.